The manufacture of integrated circuits involves the transfer of geometric shapes on a mask to the surface of a semiconductor wafer. Thereafter, the semiconductor wafer corresponding to the geometric shapes, or corresponding to the areas between the geometric shapes, is etched away. The transfer of the shapes from the mask to the semiconductor wafer typically involves a lithographic process. This includes applying a solution of a pre-polymer solution to the semiconductor wafer, the pre-polymer being selected to form a radiation-sensitive polymer which reacts when exposed to ultraviolet light, electron beams, x-rays, or ion beams, for example. The solvent in the pre-polymer solution is removed by evaporation, and the resulting polymer film is then baked. The film is exposed to radiation, for example, ultraviolet light, through a photomask supporting the desired geometric patterns. The images in the photosensitive material are then developed by soaking the wafer in a developing solution. The exposed or unexposed areas are removed in the developing process, depending on the nature of the radiation-sensitive material. Thereafter, the wafer is placed in an etching environment which etches away the areas not protected by the radiation-sensitive material. Due to their resistance to the etching process, the radiation sensitive-materials are also known as photoresists, and the term photoresist is used hereinafter to denote the radiation-sensitive polymers and their pre-polymers.
The photoresist film thickness required depends on the desired resolution, defect protection, and step coverage. Thicker films provide better adhesion, greater protection for reactive ion erosion, and improved defect protection. However, thicker films also result in lower resolution because they take longer to expose and develop. Photoresist film thicknesses used in current semiconductor manufacturing may be typically 0.5 to 4 .mu.m thick.
Thickness uniformity of the photoresist layer is an important criterion in the manufacture of integrated circuits. When the radiation is focused through the mask onto the coating, variations in thickness of the coating prevent the precise focus over the entire surface of the wafer which is required to obtain the sharpness necessary to ensure satisfactory reproduction of the geometric patterns on the semiconductor wafer for advanced circuits with line width dimensions approaching 0.25 .mu.m line widths and smaller over a surface. Photoresist film thickness uniformity is required to maintain good transfer of the mask pattern to the photoresist. Uniformity is important to maintain a constant exposure level across the surface of the wafer. Nonuniformities cause position overlay errors when optical steppers attempt to sense alignment marks beneath the photoresist film. Nonuniformities also change the reflectivity of a photoresist deposited over an oxide.
The small critical dimensions of microelectronic devices require photoresist coating thickness typically to be uniform to within 10 .ANG. (3.sigma.). As the critical dimension decreases further, even better uniformities will be required.
The high cost of the photoresist pre-polymer solutions makes it desirable to devise methods of improving the efficiency of the coating process so as to minimize the amount of the polymer solution required to coat a substrate.
Methods which have been used or proposed for coating wafers include dip coating, meniscus coating, spray coating, patch coating, bubble coating, chemical vapor deposition, and spin coating. Only a few of these methods produce photoresist films with the thicknesses and uniformities required for semiconductor production. Of these methods, only spin coating has a production rate fast enough to meet the demands of chip manufacturers. One major shortcoming of spin coating, however, is that it can waste as much as 90%, or more, of the photoresist applied to the wafer surface.
About one million gallons of photoresist are consumed each year at a cost of several hundred million dollars. As the critical dimension of semiconductor devices becomes smaller, new deep UV photoresists will be used. These new photoresists can cost five or more times the cost of the i-line photoresists used currently. Therefore, a new coating method is needed which wastes less photoresist while producing uniform, defect-free coatings at a rate comparable to that of spin coating.